German patent application no. DE 100 31 461.9 A1 discloses a high-voltage diode. In the case of such high-voltage diodes, often used as free-wheeling diodes, a soft turn-off behavior is desired in order to avoid the so-called “chopping” of the component during the turn-off process, since this effect can entail destruction of the power semiconductor. Component chopping has been avoided hitherto either by choosing a sufficiently large thickness for the component and/or by means of an anodal local lifetime setting.
The accompanying FIG. 1 schematically shows the structure of the known high-voltage diode in the form of a central cross section. Such a high-voltage diode comprises a silicon body. Instead of silicon, it is also possible to choose another suitable semiconductor material, such as e.g. SiC, etc. The silicon body has an n−conducting drift zone 1, a p-conducting zone 2, which forms a pn junction 3 with the n-conducting drift zone, a p+-conducting anode emitter 4, an n-conducting zone 5 and an n+-conducting cathode emitter 6 formed therein. On the front side V, the anode emitter 4 is provided with an anode metallization 7. On the rear side R of the component, the cathode emitter 6 is provided with a cathode metallization 8. Known contact materials such as e.g. aluminum, AlSi, etc. may be chosen for said metallization. It should be mentioned that the dimensions of the known high-voltage diode shown in FIG. 1 serve merely for elucidation purposes and do not represent the actual relationships.
In the case of the high-voltage diode disclosed in this document, the setting of the charge carrier lifetime takes place by means of crystal damage near the surface in the anode emitter or in the cathode emitter.
For optimizing the turn-off behavior of a high-voltage diode, the procedure heretofore has been such that the charge carrier lifetime has been reduced exclusively in the p+-conducting anode emitter. FIG. 2 illustrates a simulation of the resulting relationships of the doping concentration (curve A) and the defect concentration (curve B) of a high-voltage diode influenced in this way, to be precise relative to the depth y from the front side V here, y starting with the top side of the p+-type anode emitter. FIG. 2 shows that a large local increase in the defect concentration (curve B) occurs in the p+-type anode emitter approximately at a depth y of between 15 and 18 μm from the top side thereof, which locally reduces the charge carrier lifetime (τ) there.
In the case of such high-voltage diodes, it is always necessary to find a compromise between the “soft recovery behavior” of the component, the switching losses and the forward voltage drop or the on-state losses. This requirement is not taken into account by the known method for optimizing the turn-off-method as explained with reference to FIG. 2 since, although it obtains a soft turn-off behavior, this is at the expense of a higher forward voltage drop or an increased on-state power loss.
Humble O. et al.: “4.5 Kilovolt-Fast Diodes with Expanded SOA Using a Multi-Energy Proton Life Time Control Technique”, 11th International Symposium on Power Semiconductor Devices and ICs, ISPSD 99. Proceedings, Toronto, May 26-28, 1999, International Symposium on Power Semiconductor Devices and ICs, New York, N.Y.: IEEE, US, May 26, 1999, May 26, 1999 pages 121-124, XP000903559 ISBN: 0-78-03-5291-2, describes a power diode with setting of the emitter efficiency and charge carrier lifetime both in the cathode region of the diode and in the n-type base region, to be precise by irradiation with proton beams or α-particles. FIG. 3 in the right-hand column of page 121 of this document shows that although the first proton peak lies in the cathode region of the diode, the second proton peak lies near the center of the n-type base.